Although this application relates to measurement of environmental changes using cavity ring-down assisted surface plasmon detection, the following background in absorption spectroscopy may be useful in understanding the present invention.
Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIG. 1 illustrates the electromagnetic spectrum on a logarithmic scale. The science of spectroscopy studies spectra. In contrast with sciences concerned with other parts of the spectrum, visible optics particularly involves visible and near-visible light—a very narrow part of the available spectrum which extends in wavelength from about 1 mm to about 1 nm. Near visible light includes wavelengths slightly longer than red (infrared) and wavelengths slightly shorter than violet (ultraviolet). The range extends just far enough to either side of human visibility that the light can still be handled by most lenses and mirrors made of materials commonly used for visible optics. The wavelength dependence of optical properties of materials must often be considered to ensure that the optical elements formed of these materials have the desired effects.
Absorption-type spectroscopy offers high sensitivity, response times on the order of microseconds, immunity from poisoning, and limited interference from molecular species other than the species under study. Various molecular species may be detected or identified by absorption spectroscopy. Thus, absorption spectroscopy provides a general method of detecting important trace species. In the gas phase, the sensitivity and selectivity of this method is optimized because the species have their absorption strength concentrated in a set of sharp spectral lines. The narrow lines in the spectrum can be used to discriminate against most interfering species.
In many industrial processes, it is desirable to measure and analyze the concentration of trace species in flowing gas streams and liquids with a high degree of speed and accuracy. Such measurement and analysis is required when the concentration of contaminants is critical to the quality of the end product, but may still be desirable even when not required. For example, gases such as N2, O2, H2, Ar, and He are used to manufacture integrated circuits, for example, and the presence in those gases of impurities—even at parts per billion (ppb) levels—may prove damaging and reduce the yield of operational circuits. Therefore, the relatively high sensitivity with which water and other potential contaminants can be spectroscopically monitored is important to manufacturers of high-purity gases used in the semiconductor industry. These and various other impurities must be detected in many other industrial applications, as well.
Further, the presence of impurities, either inherent or deliberately released, in fluids of all kinds have become of particular concern recently. Spectroscopic methods provide a convenient means to monitor fluids such as gases and liquids (i.e. air and water) for contamination by hazardous chemical and biological agents. These methods may also be used for detection of chemical signatures of materials such as explosives and drugs.
In all of these applications, sensitivity is an important concern for any detection method. Spectroscopy has obtained parts per million (ppm) level detection for gaseous contaminants in high-purity gases. Detection sensitivities at the ppb level are attainable in some cases. Accordingly, several spectroscopic methods have been applied to such applications as quantitative contamination monitoring in gases, including: absorption measurements in traditional long path length cells, photoacoustic spectroscopy, frequency modulation spectroscopy, and intracavity laser absorption spectroscopy. Unfortunately, these methods have several features, discussed in U.S. Pat. No. 5,528,040 issued to Lehmann, that have made them difficult to use and impractical for industrial applications. They have been largely confined, therefore, to laboratory investigations.
In contrast, cavity ring-down spectroscopy (CRDS) has become an important spectroscopic technique with applications to science, industrial process control, and atmospheric trace gas detection. CRDS has been demonstrated as a technique for the measurement of optical absorption that excels in the low-absorbance regime where conventional methods have inadequate sensitivity. CRDS utilizes the mean lifetime of photons in a high-finesse optical resonator as the absorption-sensitive observable.
Typically, the resonator includes a pair of nominally equivalent, narrow band, ultra-high reflectivity dielectric mirrors, configured appropriately to form a stable standing wave optical cavity, or resonator. A laser pulse is injected into the resonator through one of the mirrors to experience a mean lifetime which depends upon the photon round-trip transit time, the length of the resonator, the absorption cross section and number density of the species being detected, and a factor accounting for intrinsic resonator losses (which arise largely from the frequency-dependent mirror reflectivities when diffraction losses are negligible). The determination of optical absorption is transformed, therefore, from the conventional power-ratio measurement to a measurement of decay time. The ultimate sensitivity of CRDS is determined by the magnitude of the intrinsic resonator losses, which can be minimized with techniques such as superpolishing that permit the fabrication of ultra-low-loss optics.
At present, CRDS is limited to spectroscopic regions where high reflectivity dielectric mirrors are produced. This has significantly limited the usefulness of the method in much of the ultraviolet and infrared regions, because mirrors with sufficiently high reflectivity are not presently available. Even in regions where suitable dielectric mirrors are available, each set of mirrors only allows for operation over a small range of wavelengths, typically a fractional range of a few percent. Further, construction of many dielectric mirrors requires use of materials that may degrade over time, especially when exposed to chemically corrosive environments. Because these present limitations restrict or prevent the use of CRDS in many potential applications, there is a clearly recognized need to improve upon the current state of the art with respect to resonator construction.
When light impinges on a surface of lower index of refraction that the propagation medium at greater than a critical angle, it reflects completely, i.e. it exhibits total internal reflection (TIR). J. D. Jackson, “Classical Electrodynamics,” Chapter 7, John Wiley & Sons, Inc.: New York, N.Y. (1962). A field exists, however, beyond the point of reflection that is non-propagating and decays exponentially with distance from the interface. This evanescent field carries no power in a pure dielectric medium, but attenuation of the reflected wave allows observation of the presence of an absorbing species in the region of the evanescent field. F. M. Mirabella (ed.), “Internal Reflection Spectroscopy,” Chapter 2, Marcel Dekker, Inc.: New York, N.Y. (1993). The article by A. Pipino et al., “Evanescent wave cavity ring-down spectroscopy with a total-internal reflection minicavity,” Rev. Sci. Instrum. 68 (8) (August 1997), presents an approach to improved resonator construction using TIR. This approach uses a monolithic, TIR ring resonator (i.e. a traveling wave optical cavity) of regular polygonal geometry (e.g., square and octagonal) with at least one convex facet to induce stability. A light pulse is totally reflected by a first prism located outside and in the vicinity of the resonator, creating an evanescent wave which enters the resonator and excites the stable modes of the resonator through photon tunneling.
The absorption spectrum of matter located at the totally reflecting surfaces of this resonator is obtained from the mean lifetime of a photon in the monolithic resonator, which is extracted from the time dependence of the signal received at a detector by out coupling with a second prism (also a totally reflecting prism located outside, but in the vicinity of, the resonator). Thus, optical radiation enters and exits the resonator by photon tunneling, which permits precise control of input and output coupling. A miniature-resonator realization of CRDS results and the TIR-ring resonator extends the CRDS concept to condensed matter spectroscopy. The broadband nature of TIR circumvents the narrow bandwidth restriction imposed by dielectric mirrors in conventional gas-phase CRDS. It is noted that the work of A. Pipino et al. is only applicable to TIR spectroscopy, which is intrinsically limited to short overall absorption path lengths, and thus powerful absorption strengths.
Various novel approaches to mirror based CRDS systems are provided in U.S. Pat. Nos. 5,973,864; 6,097,555; 6,172,823 B1; and 6,172,824 B1 issued to Lehmann et al., and incorporated herein by reference. These approaches teach the use of a near-confocal resonator formed by two reflecting elements or prismatic elements.
FIG. 2 illustrates prior art CRDS detector 10 in which ring down cavity (RDC) cell 60 is shown a standing wave configuration. As shown in FIG. 2, light is generated by narrow band, tunable, continuous wave diode laser 20. Laser 20 may be temperature tuned by a temperature controller 30 to adjust its wavelength to the desired spectral line of the analyte. An isolator 40 is positioned in front of and in line with the radiation emitted from laser 20. Isolator 40 provides a one-way transmission path, allowing radiation to travel away from laser 20 while preventing radiation from traveling in the opposite direction. Such an isolator desirably reduces noise in laser 20 caused by unwanted reflection or scattering of light back into the laser cavity. Single mode fiber coupler (F.C.) 50 couples the light emitted from laser 20 into the optical fiber 48. Fiber coupler 50 is positioned in front of and in line with isolator 40. Fiber coupler 50 receives and holds optical fiber 48 and directs the radiation emitted from laser 20 toward and through a first lens 46. First lens 46 collects and focuses the radiation. Because the beam pattern emitted by laser 20 does not perfectly match the pattern of light propagating in optical fiber 48, there is an inevitable mismatch loss. It is noted that free space optics may be used alternatively to transmit the laser light.
The laser radiation is approximately mode-matched into RDC cell 60. A reflective mirror 52 directs the radiation toward a beam splitter 54. Beam splitter 54 directs about 90%, of the radiation through a second lens 56. Second lens 56 collects and focuses the radiation into cell 60. The remaining radiation passes through beam splitter 54 and is directed by a reflective mirror 58 into an analyte reference cell 90.
The radiation which is transmitted through analyte reference cell 90 is directed toward and through a fourth lens 92. Fourth lens 92 is aligned between analyte reference cell 90 and a second photodetector 94 (PD 2). Photodetector 94 provides input to computer and control electronics 100.
Cell 60 is made from two, highly reflective mirrors 62, 64, which are aligned as a near confocal etalon along an axis, a. Mirrors 62, 64 constitute the input and output windows of cell 60. The sample gas under study flows through a narrow tube 66 that is coaxial with the optical axis, a, of cell 60. Mirrors 62, 64 are placed on adjustable flanges or mounts that are sealed with vacuum tight bellows to allow adjustment of the optical alignment of cell 60.
Mirrors 62, 64 have a high-reflectivity dielectric coating and are oriented with the coating facing inside the cavity formed by cell 60. A small fraction of laser light enters cell 60 through front mirror 62 and “rings” back and forth inside the cavity of cell 60. Light transmitted through rear mirror 64 (the reflector) of cell 60 is directed toward and through a third lens 68 and, in turn, imaged onto a first photodetector 70 (PD 1). Each of photodetectors 70, 94 converts an incoming optical beam into an electrical current and, therefore, provides an input signal to computer and control electronics 100. The input signal represents the decay rate of the cavity ring down.
FIG. 3 illustrates optical path within prior art prism based CRDS resonator 100 which is designed to operate in a traveling wave configuration. As shown in FIG. 3, resonator 100 for CRDS is based upon using two Brewster's angle retroreflector prisms 150 and 152. The polarizing or Brewster's angle, ΘB, is shown relative to prism 150. Incident light 12 and exiting light 14 are illustrated as input to and output from prism 152, respectively. The resonant optical beam undergoes two total internal reflections without loss in each prism 150 and 152 at about 45°, an angle which is greater than the critical angle for fused quartz and most other common optical prism materials within the visible spectrum. Light travels between prisms 150 and 152 along optical axis 154. Alternatively, three or more high reflectivity mirrors may be used to form mirror based traveling wave RDC. A traveling wave RDC, such as prism based CRDS resonator 100, may be used in the place of standing wave RDC 60 in CRDS detector 10 shown in FIG. 2.
In both of the traveling wave RDC's described precise alignment of the prisms, or mirrors, with each other and with the input and output beams is necessary. Precise tuning of the distance between mirrors 62 and 64 may also be desirable in standing wave RDC 60 to allow the laser light to resonate within the optical cavity. This means that these cavities may be adversely affected by environmental changes such as changes in temperature or refractive index of the medium in the cavity.
As described by the inventors in pending application Ser. No. 10/644,137 filed on Aug. 20, 2003, and its predecessors Ser. No. 10/157,400 filed on May 29, 2002, and Ser. No. 10/017,367 filed on Dec. 12, 2001, from which the present application proceeds, the use of a passive fiber optic ring resonator in a CRDS detector may prove useful in overcoming at least some of the difficulties of using prior art RDC's such as those illustrated in FIGS. 2 and 3. The present invention utilizes surface plasmon resonance to improve the sensitivity of CRDS detectors.